Thermal conductive sheet

ABSTRACT

A thermal conductive sheet containing a plate-like boron nitride particle, wherein the thermal conductivity in a direction perpendicular to the thickness direction of the thermal conductive sheet is 4 W/m·K or more, and a glass transition point determined as the peak value of tanδ obtained by measuring a dynamic viscoelasticity of the thermal conductive sheet at a frequency of 10 Hz is 125° C. or more.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority from Japanese Patent ApplicationNo. 2010-018256 filed on Jan. 29, 2010; No. 2010-090908 filed on Apr. 9,2010; and No. 2010-161853 filed on Jul. 16, 2010, the contents of whichare hereby incorporated by reference into this application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a thermal conductive sheet, to bespecific, to a thermal conductive sheet for use in power electronicstechnology.

2. Description of Related Art

In recent years, power electronics technology which uses semiconductorelements to convert and control electric power is applied in hybriddevices, high-brightness LED devices, and electromagnetic inductionheating devices. In power electronics technology, a high current isconverted to, for example, heat, and therefore materials that aredisposed near the semiconductor element are required to have excellentheat dissipation characteristics (excellent heat conductivity).

For example, Japanese Unexamined Patent Publication No. 2008-280496 hasproposed a thermal conductive sheet containing a plate-like boronnitride powder and an acrylic ester copolymer resin.

In the thermal conductive sheet of Japanese Unexamined PatentPublication No. 2008-280496, the boron nitride powder is oriented so asto orient its major axis direction (direction perpendicular to the platethickness of the boron nitride powder) in the thickness direction of thesheet, and thermal conductivity in the thickness direction of thethermal conductive sheet is improved in this way.

SUMMARY OF THE INVENTION

However, there are cases where the thermal conductive sheet is requiredto have a high thermal conductivity in a direction (plane direction)perpendicular to the thickness direction depending on its use andpurpose. In such a case, the thermal conductive sheet of JapaneseUnexamined Patent Publication No. 2008-280496 is disadvantageous in thatthe major axis direction of the boron nitride powder is perpendicular to(crossing) the plane direction, and therefore the thermal conductivityin the plane direction is insufficient.

Furthermore, when such a thermal conductive sheet is used, for example,the thermal conductive sheet is bonded to various devices for conducting(dissipating) heat generated from the devices, and therefore excellentheat resistance (resistance to deformation) is required so as not to bedeformed by the heat and not to be peeled off from the devices.

An object of the present invention is to provide a thermal conductivesheet that is excellent in thermal conductivity in the plane direction,and also excellent in heat resistance.

A thermal conductive sheet of the present invention contains aplate-like boron nitride particle, wherein the thermal conductivity in adirection perpendicular to the thickness direction of the thermalconductive sheet is 4 W/m·K or more, and a glass transition pointdetermined as the peak value of tanδ obtained by measuring a dynamicviscoelasticity of the thermal conductive sheet at a frequency of 10 Hzis 125° C. or more.

The thermal conductive sheet of the present invention is excellent inthermal conductivity in the plane direction that is perpendicular to thethickness direction, and also excellent in heat resistance.

Therefore, the thermal conductive sheet of the present invention can beapplied to various heat dissipation uses as a thermal conductive sheetthat allows decrease in deformation under high temperature, thatsuppresses peeling off, that is excellent in handleability, and that isexcellent in thermal conductivity in the plane direction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a perspective view of an embodiment of a thermal conductivesheet of the present invention.

FIG. 2 shows process drawings for describing a method for producing thethermal conductive sheet shown in FIG. 1:

(a) illustrating a step of hot pressing a mixture or a laminated sheet,

(b) illustrating a step of dividing the pressed sheet into a pluralityof pieces, and

(c) illustrating a step of laminating the divided sheets.

FIG. 3 shows a perspective view of a test device (Type I, before bendtest) of a bend test.

FIG. 4 shows a perspective view of a test device (Type I, during bendtest) of a bend test.

DETAILED DESCRIPTION OF THE INVENTION

A thermal conductive sheet of the present invention contains boronnitride particles.

To be specific, the thermal conductive sheet contains boron nitride (BN)particles as an essential component, and further contains, for example,a resin component.

The boron nitride particles are formed into a plate-like (or flake-like)shape, and are dispersed so as to be orientated in a predetermineddirection (described later) in the thermal conductive sheet.

The boron nitride particles have an average length in the longitudinaldirection (maximum length in the direction perpendicular to the platethickness direction) of, for example, 1 to 100 μm, or preferably 3 to 90μm. The boron nitride particles have an average length in thelongitudinal direction of, 5 μm or more, preferably 10 μm or more, morepreferably 20 μm or more, even more preferably 30 μm or more, or mostpreferably 40 μm or more, and usually has an average length in thelongitudinal direction of, for example, 100 μm or less, or preferably 90μm or less.

The average thickness (the length in the thickness direction of theplate, that is, the length in the short-side direction of the particles)of the boron nitride particles is, for example, 0.01 to 20 μm, orpreferably 0.1 to 15 μm.

The aspect ratio (length in the longitudinal direction/thickness) of theboron nitride particles is, for example, 2 to 10000, or preferably 10 to5000.

The average particle size of the boron nitride particles as measured bya light scattering method is, for example, 5 μm or more, preferably 10μm or more, more preferably 20 μm or more, particularly preferably 30 μmor more, or most preferably 40 μm or more, and usually is 100 μm orless.

The average particle size as measured by the light scattering method isa volume average particle size measured with a dynamic light scatteringtype particle size distribution analyzer.

When the average particle size of the boron nitride particles asmeasured by the light scattering method is below the above-describedrange, the thermal conductive sheet may become fragile, andhandleability may be reduced.

The bulk density (JIS K 5101, apparent density) of the boron nitrideparticles is, for example, 0.3 to 1.5 g/cm³, or preferably 0.5 to 1.0g/cm³.

As the boron nitride particles, a commercially available product orprocessed goods thereof can be used. Examples of commercially availableproducts of the boron nitride particles include the “PT” series (forexample, “PT-110”) manufactured by Momentive Performance Materials Inc.,and the “SHOBN®UHP” series (for example, “SHOBN®UHP-1”) manufactured byShowa Denko K.K.

The resin component is a component that is capable of dispersing theboron nitride particles, i.e., a dispersion medium (matrix) in which theboron nitride particles are dispersed, including, for example, resincomponents such as a thermosetting resin component and a thermoplasticresin component.

Examples of the thermosetting resin component include epoxy resin,thermosetting polyimide, phenol resin, urea resin, melamine resin,unsaturated polyester resin, diallyl phthalate resin, silicone resin,and thermosetting urethane resin.

Examples of the thermoplastic resin component include polyolefin (forexample, polyethylene, polypropylene, and ethylene-propylene copolymer),acrylic resin (for example, polymethyl methacrylate), polyvinyl acetate,ethylene-vinyl acetate copolymer, polyvinyl chloride, polystyrene,polyacrylonitrile, polyamide (Nylon®), polycarbonate, polyacetal,polyethylene terephthalate, polyphenylene oxide, polyphenylene sulfide,polysulfone, polyether sulfone, poly ether ether ketone, polyallylsulfone, thermoplastic polyimide, thermoplastic urethane resin,polyamino-bismaleimide, polyamide-imide, polyether-imide,bismaleimide-triazine resin, polymethylpentene, fluorine resin, liquidcrystal polymer, olefin-vinyl alcohol copolymer, ionomer, polyarylate,acrylonitrile-ethylene-styrene copolymer,acrylonitrile-butadiene-styrene copolymer, and acrylonitrile-styrenecopolymer.

These resin components can be used alone or in combination of two ormore.

Of the thermosetting resin components, preferably, an epoxy resin isused.

The epoxy resin is in a state of liquid, semi-solid, or solid undernormal temperature.

To be specific, examples of the epoxy resin include aromatic epoxyresins such as bisphenol epoxy resin (for example, bisphenol A epoxyresin, bisphenol F epoxy resin, bisphenol S epoxy resin, hydrogenatedbisphenol A epoxy resin, dimer acid-modified bisphenol epoxy resin, andthe like), novolak epoxy resin (for example, phenol novolak epoxy resin,cresol novolak epoxy resin, biphenyl epoxy resin, and the like),naphthalene epoxy resin, fluorene epoxy resin (for example, bisarylfluorene epoxy resin and the like), and triphenylmethane epoxy resin(for example, trishydroxyphenylmethane epoxy resin and the like);nitrogen-containing-cyclic epoxy resins such as triepoxypropylisocyanurate (triglycidyl isocyanurate) and hydantoin epoxy resin;aliphatic epoxy resin; alicyclic epoxy resin (for example, dicycloring-type epoxy resin and the like); glycidylether epoxy resin; andglycidylamine epoxy resin.

These epoxy resins can be used alone or in combination of two or more.

The epoxy resin has an epoxy equivalent of, for example, 100 to 1000g/eqiv., or preferably 180 to 700 g/eqiv., and has a softeningtemperature (ring and ball test) of, for example, 80° C. or less (to bespecific, 20 to 80° C.), or preferably 70° C. or less (to be specific,35 to 70° C.).

The epoxy resin has a melt viscosity at 80° C. of, for example, 10 to20,000 mPa·s, or preferably 50 to 10,000 mPa·s. When two or more epoxyresins are used in combination, the melt viscosity of the mixture ofthese epoxy resins is set within the above-described range.

When two or more epoxy resins are used in combination, for example, acombination of a liquid epoxy resin and a solid epoxy resin is used, ormore preferably, a combination of a liquid aromatic epoxy resin and asolid aromatic epoxy resin is used. Examples of such a combinationinclude, to be more specific, a combination of a liquid bisphenol epoxyresin and a solid triphenylmethane epoxy resin, and a combination of aliquid bisphenol epoxy resin and a solid bisphenol epoxy resin.

Preferably, a semi-solid epoxy resin is used alone, or more preferably,a semi-solid aromatic epoxy resin is used alone. Examples of those epoxyresins include, in particular, a semi-solid fluorene epoxy resin.

A combination of a liquid epoxy resin and a solid epoxy resin, or asemi-solid epoxy resin can improve conformability to irregularities(described later) of the thermal conductive sheet.

Furthermore, by combining a plurality of epoxy resins having differentproperties and condition, the glass transition point can be set to adesired range.

Furthermore, when two or more epoxy resins are used in combination, forexample, a first epoxy resin having a softening temperature of, forexample, below 45° C., or preferably 35° C. or less, and a second epoxyresin having a softening temperature of, for example, 45° C. or more, orpreferably 55° C. or more are used in combination. In this way, thekinetic viscosity (in conformity with JIS K 7233, described later) ofthe resin component (mixture) can be set to a desired range, and also,conformability to irregularities of the thermal conductive sheet can beimproved.

The epoxy resin can also be prepared as an epoxy resin compositioncontaining, for example, an epoxy resin, a curing agent, and a curingaccelerator.

The curing agent is a latent curing agent (epoxy resin curing agent)that can cure the epoxy resin by heating, and examples thereof includean imidazole compound, an amine compound, an acid anhydride compound, anamide compound, a hydrazide compound, and an imidazoline compound. Inaddition to the above-described compounds, a phenol compound, a ureacompound, and a polysulfide compound can also be used.

Examples of the imidazole compound include 2-phenyl imidazole, 2-methylimidazole, 2-ethyl-4-methyl imidazole, and2-phenyl-4-methyl-5-hydroxymethyl imidazole.

Examples of the amine compound include aliphatic polyamines such asethylene diamine, propylene diamine, diethylene triamine, andtriethylene tetramine; and aromatic polyamines such as methaphenylenediamine, diaminodiphenyl methane, and diaminodiphenyl sulfone.

Examples of the acid anhydride compound include phthalic anhydride,maleic anhydride, tetrahydrophthalic anhydride, hexahydrophthalicanhydride, 4-methyl-hexahydrophthalic anhydride, methyl nadic anhydride,pyromelletic anhydride, dodecenylsuccinic anhydride, dichloro succinicanhydride, benzophenone tetracarboxylic anhydride, and chlorendicanhydride.

Examples of the amide compound include dicyandiamide and polyamide.

An example of the hydrazide compound includes adipic acid dihydrazide.

Examples of the imidazoline compound include methylimidazoline,2-ethyl-4-methylimidazoline, ethylimidazoline, isopropylimidazoline,2,4-dimethylimidazoline, phenylimidazoline, undecylimidazoline,heptadecylimidazoline, and 2-phenyl-4-methylimidazoline.

These curing agents can be used alone or in combination of two or more.

A preferable example of the curing agent is an imidazole compound.

Examples of the curing accelerator include tertiary amine compounds suchas triethylenediamine and tri-2,4,6-dimethylaminomethylphenol;phosphorus compounds such as triphenylphosphine, tetraphenylphosphoniumtetraphenylborate, andtetra-n-butylphosphonium-o,o-diethylphosphorodithioate; a quaternaryammonium salt compound; an organic metal salt compound; and derivativesthereof. These curing accelerators can be used alone or in combinationof two or more.

In the epoxy resin composition, the mixing ratio of the curing agent is,for example, 0.5 to 50 parts by mass, or preferably 1 to 10 parts bymass per 100 parts by mass of the epoxy resin, and the mixing ratio ofthe curing accelerator is, for example, 0.1 to 10 parts by mass, orpreferably 0.2 to 5 parts by mass per 100 parts by mass of the epoxyresin.

The above-described curing agent, and/or the curing accelerator can beprepared and used, as necessary, as a solution, i.e., the curing agentand/or the curing accelerator dissolved in a solvent; and/or as adispersion liquid, i.e., the curing agent and/or the curing acceleratordispersed in a solvent.

Examples of the solvent include organic solvents including ketones suchas acetone and methyl ethyl ketone, ester such as ethyl acetate, andamide such as N,N-dimethylformamide; and water-based solvents includingwater, and alcohols such as methanol, ethanol, propanol, andisopropanol. A preferable example is an organic solvent, more preferableexamples are ketones and amides.

Of the thermoplastic resin components, preferably, polyolefin is used.

Preferable examples of polyolefin are polyethylene andethylene-propylene copolymer.

Examples of polyethylene include a low density polyethylene and a highdensity polyethylene.

Examples of ethylene-propylene copolymer include a random copolymer, ablock copolymer, or a graft copolymer of ethylene and propylene.

These polyolefins can be used alone or in combination of two or more.

The polyolefins have a weight average molecular weight and/or a numberaverage molecular weight of, for example, 1000 to 10000.

The polyolefin can be used alone, or can be used in combination.

Of the resin components, preferably, a thermosetting resin component isused, or more preferably, an epoxy resin is used.

The resin component has a kinetic viscosity as measured in conformitywith the kinetic viscosity test of JIS K 7233 (bubble viscometer method)(temperature: 25° C.±0.5° C., solvent: butyl carbitol, resin component(solid content) concentration: 40 mass %) of, for example, 0.22×10⁻⁴ to2.00×10⁻⁴ m²/s, preferably 0.3×10⁻⁴ to 1.9×10⁻⁴ m²/s, or more preferably0.4×10⁻⁴ to 1.8×10⁻⁴ m²/s. The above-described kinetic viscosity canalso be set to, for example, 0.22×10⁻⁴ to 1.00×10⁻⁴ m²/s, preferably0.3×10⁻⁴ to 0.9×10⁻⁴ m²/s, or more preferably 0.4×10⁻⁴ to 0.8×10⁻⁴ m²/s.

When the kinetic viscosity of the resin component exceeds theabove-described range, excellent flexibility and conformability toirregularities (described later) may not be given to the thermalconductive sheet. On the other hand, when the kinetic viscosity of theresin component is below the above-described range, boron nitrideparticles may not be oriented in a predetermined direction.

In the kinetic viscosity test in conformity with JIS K 7233 (bubbleviscometer method), the kinetic viscosity of the resin component ismeasured by comparing the bubble rising speed of a resin componentsample with the bubble rising speed of criterion samples (having a knownkinetic viscosity), and determining the kinetic viscosity of thecriterion sample having a matching rising speed to be the kineticviscosity of the resin component.

In the thermal conductive sheet, the proportion of the volume-basedboron nitride particle content (solid content, that is, the volumepercentage of boron nitride particles relative to a total volume of theresin component and the boron nitride particles) is, for example, 35 vol% or more, preferably 60 vol % or more, or more preferably 75 vol % ormore, and usually, for example, 95 vol % or less, or preferably 90 vol %or less.

When the proportion of the volume-based boron nitride particle contentis below the above-described range, the boron nitride particles may notbe oriented in a predetermined direction in the thermal conductivesheet. On the other hand, when the proportion of the volume-based boronnitride particle content exceeds the above-described range, the thermalconductive sheet may become fragile, and handleability andconformability to irregularities may be reduced.

The mass-based mixing ratio of the boron nitride particles relative to100 parts by mass of the total amount (total solid content) of thecomponents (boron nitride particles and resin component) forming thethermal conductive sheet is, for example, 40 to 95 parts by mass, orpreferably 65 to 90 parts by mass, and the mass-based mixing ratio ofthe resin component relative to 100 parts by mass of the total amount ofthe components forming the thermal conductive sheet is, for example, 5to 60 parts by mass, or preferably 10 to 35 parts by mass. Themass-based mixing ratio of the boron nitride particles relative to 100parts by mass of the resin component is, for example, 60 to 1900 partsby mass, or preferably 185 to 900 parts by mass.

When two epoxy resins (a first epoxy resin and a second epoxy resin) areused in combination, the mass ratio (mass of the first epoxy resin/massof the second epoxy resin) of the first epoxy resin relative to thesecond epoxy resin can be set appropriately in accordance with thesoftening temperature and the like of the epoxy resins (the first epoxyresin and the second epoxy resin). For example, the mass ratio of thefirst epoxy resin relative to the second epoxy resin is 1/99 to 99/1, orpreferably 10/90 to 90/10.

In the resin component, in addition to the above-described components(polymer), for example, a polymer precursor (for example, a lowmolecular weight polymer including oligomer), and/or a monomer arecontained.

FIG. 1 shows a perspective view of an embodiment of a thermal conductivesheet of the present invention, and FIG. 2 shows process drawings fordescribing a method for producing the thermal conductive sheet shown inFIG. 1.

Next, a method for producing a thermal conductive sheet as an embodimentof the present invention is described with reference to FIG. 1 and FIG.2.

In this method, first, the above-described components are blended at theabove-described mixing ratio and are stirred and mixed, therebypreparing a mixture.

In the stirring and mixing, in order to mix the components efficiently,for example, the solvent may be blended therein with the above-describedcomponents, or, for example, the resin component (preferably, thethermoplastic resin component) can be melted by heating.

Examples of the solvent include the above-described organic solvents.When the above-described curing agent and/or the curing accelerator areprepared as a solvent solution and/or a solvent dispersion liquid, thesolvent of the solvent solution and/or the solvent dispersion liquid canalso serve as a mixing solvent for the stirring and mixing withoutadding a solvent during the stirring and mixing. Alternatively, in thestirring and mixing, a solvent can be further added as a mixing solvent.

In the case when the stirring and mixing is performed using a solvent,the solvent is removed after the stirring and mixing.

To remove the solvent, for example, the mixture is allowed to stand atroom temperature for 1 to 48 hours; heated at 40 to 100° C. for 0.5 to 3hours; or heated under a reduced pressure atmosphere of, for example,0.001 to 50 kPa, at 20 to 60° C., for 0.5 to 3 hours.

When the resin component (preferably, a thermoplastic resin component)is to be melted by heating, the heating temperature is, for example, atemperature in the neighborhood of or exceeding the softeningtemperature of the resin component, to be specific, 40 to 150° C., orpreferably 70 to 140° C.

Next, in this method, the obtained mixture is hot-pressed.

To be specific, as shown in FIG. 2 (a), as necessary, for example, themixture is hot-pressed with two releasing films 4 sandwiching themixture, thereby producing a pressed sheet 1A. Conditions for thehot-pressing are as follows: a temperature of, for example, 50 to 150°C., or preferably 60 to 140° C.; a pressure of, for example, 1 to 100MPa, or preferably 5 to 50 MPa; and a duration of, for example, 0.1 to100 minutes, or preferably 1 to 30 minutes.

More preferably, the mixture is hot-pressed under vacuum. The degree ofvacuum in the vacuum hot-pressing is, for example, 1 to 100 Pa, orpreferably 5 to 50 Pa, and the temperature, the pressure, and theduration are the same as those described above for the hot-pressing.

When the temperature, the pressure, and/or the duration in thehot-pressing is outside the above-described range, there is a case wherea porosity P (described later) of the thermal conductive sheet 1 cannotbe adjusted to a desired value.

The pressed sheet 1A obtained by the hot-pressing has a thickness of,for example, 50 to 1000 μm, or preferably 100 to 800 μm.

Next, in this method, as shown in FIG. 2 (b), the pressed sheet 1A isdivided into a plurality of pieces (for example, four pieces), therebyproducing a divided sheet 1B (dividing step). In the division of thepressed sheet 1A, the pressed sheet 1A is cut along the thicknessdirection so that the pressed sheet 1A is divided into a plurality ofpieces when the pressed sheet 1A is projected in the thicknessdirection. The pressed sheet 1A is cut so that the respective dividedsheets 1B have the same shape when the divided sheets 1B are projectedin the thickness direction.

Next, in this method, as shown in FIG. 2 (c), the respective dividedsheets 1B are laminated in the thickness direction, thereby producing alaminated sheet 1C (laminating step).

Thereafter, in this method, as shown in FIG. 2 (a), the laminated sheet1C is hot-pressed (preferably hot-pressed under vacuum) (hot-pressingstep). The conditions for the hot-pressing are the same as theconditions for the hot-pressing of the above-described mixture.

The thickness of the hot-pressed laminated sheet 1C is, for example, 1mm or less, or preferably 0.8 mm or less, and usually is, for example,0.05 mm or more, or preferably 0.1 mm or more.

Thereafter, the series of the steps of the above-described dividing step(FIG. 2 (b)), laminating step (FIG. 2 (c)), and hot-pressing step (FIG.2 (a)) are performed repeatedly, so as to allow boron nitride particles2 to be efficiently oriented in a predetermined direction in the resincomponent 3 in the thermal conductive sheet 1. The number of therepetition is not particularly limited, and can be set appropriatelyaccording to the charging state of the boron nitride particles. Thenumber of the repetition is, for example, 1 to 10 times, or preferably 2to 7 times.

The thermal conductive sheet 1 can be obtained in this manner.

The thickness of the obtained thermal conductive sheet 1 is, forexample, 1 mm or less, or preferably 0.8 mm or less, and usually, forexample, 0.05 mm or more, or preferably 0.1 mm or more.

In the thermal conductive sheet 1, the proportion of the volume-basedboron nitride particle content (solid content, that is, volumepercentage of boron nitride particles relative to the total volume ofthe resin component and the boron nitride particles) is, as describedabove, for example, 35 vol % or more (preferably 60 vol % or more, ormore preferably 75 vol % or more), and usually 95 vol % or less(preferably 90 vol % or less).

When the proportion of the boron nitride particle content is below theabove-described range, the boron nitride particles may not be orientedin a predetermined direction in the thermal conductive sheet.

When the resin component 3 is the thermosetting resin component, forexample, the series of the steps of the above-described dividing step(FIG. 2 (b)), laminating step (FIG. 2 (c)), and hot-pressing step (FIG.2 (a)) are performed repeatedly for an uncured thermal conductive sheet1, and then the uncured (or semi-cured (in B-stage)) thermal conductivesheet 1 is cured by heat after the hot-pressing step (FIG. 2 (a)), i.e.,after the final step, thereby producing a cured thermal conductive sheet1.

To cure the thermal conductive sheet 1 by heat, the above-described hotpress or a dryer is used. Preferably, a dryer is used. Conditions forthe curing by heat are as follows: a heating temperature of, forexample, 60 to 250° C., or preferably 80 to 200° C., and a pressure of,for example, 100 MPa or less, or preferably 50 MPa or less.

In the thus obtained thermal conductive sheet 1, as shown in FIG. 1 andits partially enlarged schematic view, the longitudinal direction LD ofthe boron nitride particle 2 is oriented along a plane (surface)direction SD that crosses (is perpendicular to) the thickness directionTD of the thermal conductive sheet 1.

The calculated average of the angle formed between the longitudinaldirection LD of the boron nitride particle 2 and the plane direction SDof the thermal conductive sheet 1 (orientation angle α of the boronnitride particles 2 relative to the thermal conductive sheet 1) is, forexample, 25 degrees or less, or preferably 20 degrees or less, andusually 0 degree or more.

The orientation angle α of the boron nitride particle 2 relative to thethermal conductive sheet 1 is obtained as follows: the thermalconductive sheet 1 is cut along the thickness direction with a crosssection polisher (CP); the cross section thus appeared is photographedwith a scanning electron microscope (SEM) at a magnification thatenables observation of 200 or more boron nitride particles 2 in thefield of view; a tilt angle α between the longitudinal direction LD ofthe boron nitride particle 2 and the plane direction SD (directionperpendicular to the thickness direction TD) of the thermal conductivesheet 1 is obtained from the obtained SEM photograph; and the averagevalue of the tilt angles α is calculated.

The thus obtained thermal conductivity in the plane direction SD of thethermal conductive sheet 1 is 4 W/m·K or more, preferably 5 W/m·K ormore, more preferably 10 W/m·K or more, even more preferably 15 W/m·K ormore, or particularly preferably 25 W/m·K or more, and usually 200 W/m·Kor less.

The thermal conductivity in the plane direction SD of the thermalconductive sheet 1 is substantially the same before and after the curingby heat when the resin component 3 is the thermosetting resin component.

When the thermal conductivity in the plane direction SD of the thermalconductive sheet 1 is below the above-described range, thermalconductivity in the plane direction SD is insufficient, and thereforethere is a case where the thermal conductive sheet 1 cannot be used forheat dissipation that requires thermal conductivity in such a planedirection SD.

The thermal conductivity in the plane direction SD of the thermalconductive sheet 1 is measured by a pulse heating method. In the pulseheating method, the xenonflash analyzer “LFA-447” (manufactured by ErichNETZSCH GmbH & Co. Holding KG) is used.

The thermal conductivity in the thickness direction TD of the thermalconductive sheet 1 is, for example, 0.5 to 15 W/m·K, or preferably 1 to10 W/m·K.

The thermal conductivity in the thickness direction TD of the thermalconductive sheet 1 is measured by a pulse heating method, a laser flashmethod, or a TWA method. In the pulse heating method, theabove-described device is used, in the laser flash method, “TC-9000”(manufactured by Ulvac, Inc.) is used, and in the TWA method, “ai-Phasemobile” (manufactured by ai-Phase Co., Ltd) is used.

Thus, the ratio of the thermal conductivity in the plane direction SD ofthe thermal conductive sheet 1 relative to the thermal conductivity inthe thickness direction TD of the thermal conductive sheet 1 (thermalconductivity in the plane direction SD/thermal conductivity in thethickness direction TD) is, for example, 1.5 or more, preferably 3 ormore, or more preferably 4 or more, and usually 20 or less.

Although not shown in FIG. 1, for example, pores (gaps) are formed inthe thermal conductive sheet 1.

The proportion of the pores in the thermal conductive sheet 1, that is,a porosity P, can be adjusted by setting the proportion of the boronnitride particle 2 content (volume-based), and further setting thetemperature, the pressure, and/or the duration at the time of hotpressing the mixture of the boron nitride particle 2 and the resincomponent 3 (FIG. 2 (a)). To be specific, the porosity P can be adjustedby setting the temperature, the pressure, and/or the duration of the hotpressing (FIG. 2 (a)) within the above-described range.

The porosity P of the thermal conductive sheet 1 is, for example, 30 vol% or less, or preferably 10 vol % or less.

The porosity P is measured by, for example, as follows: the thermalconductive sheet 1 is cut along the thickness direction with a crosssection polisher (CP); the cross section thus appeared is observed witha scanning electron microscope (SEM) at a magnification of 200 to obtainan image; the obtained image is binarized based on the pore portion andthe non-pore portion; and the area ratio, i.e., the ratio of the poreportion area to the total area of the cross section of the thermalconductive sheet 1 is determined by calculation.

The thermal conductive sheet 1 has a porosity P2 after curing of,relative to a porosity P1 before curing, for example, 100% or less, orpreferably 50% or less.

For the measurement of the porosity P(P1), when the resin component 3 isa thermosetting resin component, the thermal conductive sheet 1 beforebeing cured by heat is used.

When the porosity P of the thermal conductive sheet 1 is within theabove-described range, the conformability to irregularities (describedlater) of the thermal conductive sheet 1 can be improved.

The glass transition point of the thermal conductive sheet 1 is 125° C.or more, preferably 130° C. or more, more preferably 140° C. or more,more preferably 150° C. or more, more preferably 170° C. or more, morepreferably 190° C. or more, or more preferably 210° C. or more, andusually 300° C. or less.

When the glass transition point is the above-described lower limit ormore, a thermal conductive sheet with excellent heat resistance can beensured, and therefore deformation under high temperature can bereduced, and peeling off can be suppressed.

That is, when the thermal conductive sheet 1 is bonded to a device ofvarious types and the temperature of the device rises to exceed theglass transition point of the thermal conductive sheet 1, the thermalconductive sheet 1 may be peeled off from the device depending on thechanges in the linear expansion coefficient. However, because the glasstransition point of this thermal conductive sheet 1 is equal to or morethan the above-described upper limit, even if the device temperaturerises, the temperature can be prevented from exceeding the glasstransition point of the thermal conductive sheet 1, and as a result,deformation of the thermal conductive sheet 1 can be reduced, and thepeeling off can be suppressed.

The glass transition point is obtained by measuring a dynamicviscoelasticity at a frequency of 10 Hz, and determining the peak valueof tanδ (loss tangent).

When the thermal conductive sheet 1 is evaluated in the bend test inconformity with the cylindrical mandrel method of JIS K 5600-5-1 underthe test conditions shown below, preferably, no fracture is observed.

Test Conditions:

Test Device: Type I

Mandrel: diameter 10 mm

Bending Angle: 90 degrees or more

Thickness of the thermal conductive sheet 1: 0.3 mm

FIGS. 3 and 4 show perspective views of the Type I test device. In thefollowing, the Type I test device is described.

In FIGS. 3 and 4, a Type I test device 10 includes a first flat plate11; a second flat plate 12 disposed in parallel with the first flatplate 11; and a mandrel (rotation axis) 13 provided for allowing thefirst flat plate 11 and the second flat plate 12 to rotate relatively.

The first flat plate 11 is formed into a generally rectangular flatplate. A stopper 14 is provided at one end portion (free end portion) ofthe first flat plate 11. The stopper 14 is formed on the surface of thesecond flat plate 12 so as to extend along the one end portion of thesecond flat plate 12.

The second flat plate 12 is formed into a generally rectangular flatplate, and one side thereof is disposed so as to be adjacent to one side(the other end portion (proximal end portion) that is opposite to theone end portion where the stopper 14 is provided) of the first flatplate 11.

The mandrel 13 is formed so as to extend along one side of the firstflat plate 11 and one side of the second flat plate 12 that are adjacentto each other.

In the Type I test device 10, as shown in FIG. 3, the surface of thefirst flat plate 11 is flush with the surface of the second flat plate12 before the start of the bend test.

To perform the bend test, the thermal conductive sheet 1 is placed onthe surface of the first flat plate 11 and the surface of the secondflat plate 12. The thermal conductive sheet 1 is placed so that one sideof the thermal conductive sheet 1 is in contact with the stopper 14.

Then, as shown in FIG. 4, the first flat plate 11 and the second flatplate 12 are rotated relatively. In particular, the free end portion ofthe first flat plate 11 and the free end portion of the second flatplate 12 are rotated to a predetermined angle with the mandrel 13 as thecenter. To be specific, the first flat plate 11 and the second flatplate 12 are rotated so as to bring the surface of the free end portionsthereof closer (oppose each other).

In this way, the thermal conductive sheet 1 is bent with the mandrel 13as the center, conforming to the rotation of the first flat plate 11 andthe second flat plate 12.

More preferably, no fracture is observed in the thermal conductive sheet1 even when the bending angle is set to 180 degrees under theabove-described test conditions.

When the resin component 3 is the thermosetting resin component, asemi-cured (in B-stage) thermal conductive sheet 1 (that is, the thermalconductive sheet 1 before being cured by heat) is tested in the bendtest.

When the fracture is observed in the bend test at the above bendingangle in the thermal conductive sheet 1, there is a case where excellentflexibility cannot be given to the thermal conductive sheet 1.

Furthermore, for example, when the thermal conductive sheet 1 isevaluated in the 3-point bending test in conformity with JIS K 7171(2008) under the test conditions shown below, no fracture is observed.

Test Conditions:

Test piece: size 20 mm×15 mm

Distance between supporting points: 5 mm

Testing speed: 20 mm/min (indenter depressing speed)

Bending angle: 120 degrees

Evaluation method: Presence or absence of fracture such as cracks at thecenter of the test piece is observed visually when tested under theabove-described test conditions.

In the 3-point bending test, when the resin component 3 is athermosetting resin component, the thermal conductive sheet 1 beforebeing cured by heat is used.

Thus, the thermal conductive sheet 1 is excellent in conformability toirregularities because no fracture is observed in the above-described3-point bending test. The conformability to irregularities is, when thethermal conductive sheet 1 is provided on an object with irregularities,a property of the thermal conductive sheet 1 that conforms to be inclose contact with the irregularities.

A mark such as, for example, letters and symbols can be given to thethermal conductive sheet 1. That is, the thermal conductive sheet 1 isexcellent in mark adhesion. The mark adhesion is a property of thethermal conductive sheet 1 that allows reliable adhesion of theabove-described mark thereon.

The mark can be adhered (applied, fixed, or firmly fixed) to the thermalconductive sheet 1, to be specific, by printing, engraving, or the like.

Examples of printing include, for example, inkjet printing, reliefprinting, intaglio printing, and laser printing.

When the mark is to be printed by inkjet printing, relief printing, orintaglio printing, for example, an ink fixing layer for improving mark'sfixed state can be provided on the surface (printing side) of thethermal conductive sheet 1.

When the mark is to be printed by laser printing, for example, a tonerfixing layer for improving mark's fixed state can be provided on thesurface (printing side) of the thermal conductive sheet 1.

Examples of engraving include laser engraving and punching.

The thermal conductive sheet 1 is excellent in thermal conductivity inthe plane direction that is perpendicular to the thickness direction,and also excellent in heat resistance.

Thus, as a thermal conductive sheet that allows decrease in deformationunder high temperature, that suppresses peeling off, that is excellentin handleability, and that has excellent thermal conductivity in theplane direction, the thermal conductive sheet can be used for variousheat dissipation applications, to be specific, as a thermal conductivesheet applied in power electronics technology, to be more specific, as athermal conductive sheet used, for example, as an LED heat dissipationsubstrate, or as a heat dissipation material for batteries.

In the above-described hot-pressing step (FIG. 2 (a)), for example, aplurality of calendering rolls and the like can also be used for rollingthe mixture and the laminated sheet 1C.

When the resin component 3 is the thermosetting resin component, withoutcuring by heat as described above, the thermal conductive sheet of thepresent invention can also be obtained as the uncured thermal conductivesheet 1.

That is, with the thermal conductive sheet of the present invention,when the resin component is the thermosetting resin component, there isno particular limitation as to whether or not curing by heat is carriedout or when curing by heat is carried out. For example, the curing byheat can be performed after the laminating step (FIG. 2 (c)) asdescribed above, or can be performed after the elapse of a predeterminedperiod from the above-described hot-pressing step (FIG. 2 (a),hot-pressing of the mixture but the hot-pressing does not allow curingby heat). To be specific, the curing by heat can be performed at thetime when the sheet is applied in power electronics technology, or afterthe elapse of a predetermined period after such application.

EXAMPLES

Hereinafter, the present invention is described in further detail withreference to Examples. However, the present invention is not limited toExamples.

Example 1

The components described below were blended, stirred, and allowed tostand at room temperature (23° C.) for one night, thereby allowingmethyl ethyl ketone (dispersion medium for the curing agent) tovolatilize, and preparing a semi-solid mixture. The details of thecomponents are as follows: 13.42 g of PT-110 (trade name, plate-likeboron nitride particles, average particle size (light scattering method)45 μm, manufactured by Momentive Performance Materials Inc.), 1.0 g ofjER®828 (trade name, bisphenol A epoxy resin, liquid, epoxy equivalent184 to 194 g/eqiv., softening temperature (ring and ball method) below25° C., melt viscosity (80° C.) 70 mPa·s, manufactured by Japan EpoxyResins Co., Ltd.), 2.0 g of EPPN-501HY (trade name, triphenylmethaneepoxy resin, solid, epoxy equivalent 163 to 175 g/eqiv., softeningtemperature (ring and ball method) 57 to 63° C., manufactured by NIPPONKAYAKU Co., Ltd), and 3.0 g (solid content 0.15 g) (5 mass % per totalamount of epoxy resins of jER®828 and EPPN-501HY) of a curing agent (asolution of 5 mass % Curezol® 2PZ (trade name, manufactured by ShikokuChemicals Corporation.) in methyl ethyl ketone).

In the above-described blending, the volume percentage (vol %) of theboron nitride particles relative to the total volume of the solidcontent excluding the curing agent (that is, solid content of the boronnitride particle and epoxy resin) was 70 vol %.

Then, the obtained mixture was sandwiched by two silicone-treatedreleasing films, and then these were hot-pressed with a vacuum hot-pressat 80° C. under an atmosphere (vacuum atmosphere) of 10 Pa with a loadof 5 tons (20 MPa) for 2 minutes. A pressed sheet having a thickness of0.3 mm was thus obtained (ref: FIG. 2 (a)).

Thereafter, the obtained pressed sheet was cut so as to be divided intoa plurality of pieces when projected in the thickness direction of thepressed sheet. Divided sheets were thus obtained (ref: FIG. 2 (b)).Next, the divided sheets were laminated in the thickness direction. Alaminated sheet was thus obtained (ref: FIG. 2 (c)).

Then, the obtained laminated sheet was hot-pressed under the sameconditions as described above with the above-described vacuum hot-press(ref: FIG. 2 (a)).

Then, a series of the above-described operations of cutting, laminating,and hot-pressing (ref: FIG. 2) was repeated four times. A thermalconductive sheet (in B-stage) having a thickness of 0.3 mm was thusobtained.

Thereafter, the obtained thermal conductive sheet was introduced into adryer, and heated at 150° C. for 120 minutes so as to be cured by heat.

Examples 2 to 14

Thermal conductive sheets were obtained in the same manner as in Example1 in accordance with the mixing formulation and production conditions ofTables 1 to 3.

(Evaluation) 1. Thermal Conductivity

The thermal conductivity of the thermal conductive sheets obtained inExamples 1 to 14 was measured.

That is, the thermal conductivity in the plane direction (SD) wasmeasured by a pulse heating method using a xenon flash analyzer“LFA-447” (manufactured by Erich NETZSCH GmbH & Co. Holding KG).

The results are shown in Tables 1 to 3.

2. Glass Transition Point

The glass transition point of the thermal conductive sheets obtained inExamples 1 to 14 was measured.

That is, the thermal conductive sheet was measured with a temperaturerising speed of 1° C./min and a frequency of 10 Hz using a dynamicviscoelasticity measuring apparatus (model number: DMS 6100,manufactured by Seiko Instruments Inc.).

The glass transition point was determined by the obtained data, i.e.,the peak value of tanδ.

The results are shown in Tables 1 to 3.

3. Porosity (P)

The porosity (P1) of the thermal conductive sheets before being cured byheat in Examples 1 to 14 was measured by the following measurementmethod.

Measurement method of porosity: The thermal conductive sheet was cutalong the thickness direction with a cross section polisher (CP); andthe cross section thus appeared was observed with a scanning electronmicroscope (SEM) at a magnification of 200. The obtained image wasbinarized based on the pore portion and the non-pore portion; and thearea ratio, i.e., the ratio of the pore portion area to the total areaof the cross section of the thermal conductive sheet was calculated.

The results are shown in Tables 1 to 3.

4. Conformability to Irregularities (3-Point Bending Test)

The 3-point bending test in conformity with JIS K 7171 (2010) wascarried out for the thermal conductive sheets before being cured by heatof Examples 1 to 14 with the following test conditions, thus evaluatingconformability to irregularities with the following evaluation criteria.The results are shown in Tables 1 to 3.

Test Conditions:

Test Piece: size 20 mm×15 mm

Distance Between Supporting Points: 5 mm

Testing Speed: 20 mm/min (indenter depressing speed)

Bending Angle: 120 degrees

(Evaluation Criteria)

Excellent: No fracture was observed.

Good: Almost no fracture was observed.

Bad: Fracture was clearly observed.

5. Printed Mark Visibility (Mark Adhesion by Printing: Mark Adhesion byInkjet Printing or Laser Printing)

Marks were printed on the thermal conductive sheets of Examples 1 to 14by inkjet printing and laser printing, and the marks were observed.

As a result, it was confirmed that the marks were excellently visible inboth cases of inkjet printing and laser printing, and that mark adhesionby printing was excellent in any of the thermal conductive sheets ofExamples 1 to 14.

TABLE 1 Average Particle Size Example (μm) Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5Ex. 6 Mixing Boron Nitride PT-110*¹ 45 13.42 3.83 5.75 12.22 23 —Formulation Particle/g*^(A)/ [70] [40] [50] [68] [80] of [vol %]*^(B)/[69] [38.8] [48.8] [66.9] [79.2] Components [vol %]*^(C) UHP-1*²  9 — —— — — 12.22 [68] [66.9] Polymer Thermosetting Epoxy Resin Epoxy ResinA*³ — 3 3 3 3 3 Resin Composition (Semi-solid) Epoxy Resin B*⁴ 1 — — — —— (Liquid) Epoxy Resin C*⁵ — — — — — — (Solid) Epoxy Resin ID*⁶ 2 — — —— — (Solid) Curing Agent*⁷ — 3 3 3 3 3 (Solid Content in (0.15) (0.15)(0.15) (0.15) (0.15) Grams) Curing Agent*⁸ 3 — — — — — (Solid Content(0.15) in Grams) Production Hot-pressing Temperature (° C.) 80 80 80 8080 80 Conditions Number of Time (times)*^(D) 5 5 5 5 5 5 Load(MPa)/(tons) 20/5 20/5 20/5 20/5 20/5 20/5 Evaluation Thermal ThermalConductivity Plane Direction 30 4.5 6.0 30.0 32.5 17.0 Conductive (W/m ·K) (SD) Sheet Thickness Direction 2.0 1.3 3.3 5.0 5.5 5.8 (TD) Ratio15.0 3.5 1.8 6.0 5.9 2.9 (SD/TD) Glass Transition Point (° C.) 216 139140 139 138 140 Porosity (vol %) 4 0 0 5 12 6 Conformability toIrregularities/3-point bending test Good Good Good Good Good Good JIS K7171 (2008) Boron Nitride Orientation Angle (α)(degrees) 18 18 18 15 1320 Particle g*^(A): Blended Weight [vol %]*^(B): Percentage relative tothe total volume of the thermal conductive sheet (excluding curingagent) [vol %]*^(C): Percentage relative to the total volume of thethermal conductive sheet Number of Time*^(D): Number of times ofhot-pressing of laminated sheet

TABLE 2 Average Particle Size Example (μm) Ex. 7 Ex. 8 Ex. 9 MixingBoron Nitride PT-110*¹ 45 12.22 12.22 13.42 Formulation Particle/g*^(A)/[68] [68] [70] of [vol %]*^(B)/ [66.9] [66.9] [69] Components [vol%]*^(C) UHP-1*²  9 — — — Polymer Thermosetting Epoxy Resin Epoxy ResinA*³ — — — Resin Composition (Semi-solid) Epoxy Resin B*⁴ 1.5 3 —(Liquid) Epoxy Resin C*⁵ 1.5 — — (Solid) Epoxy Resin D*⁶ — — 3 (Solid)Curing Agent*⁷ 3 3 3 (Solid Content in (0.15) (0.15) (0.15) Grams)Curing Agent*⁸ — — — (Solid Content in Grams) Production Hot-pressingTemperature (° C.) 80 80 80 Conditions Number of Time (times)*^(D) 5 5 5Load (MPa)/(tons) 20/5 20/5 20/5 Evaluation Thermal Thermal ConductivityPlane Direction (SD) 30.0 30.0 24.5 Conductive (W/m · K) ThicknessDirection 5.0 5.0 2.1 Sheet (TD) Ratio 6.0 6.0 11.7 (SD/TD) GlassTransition Point (° C.) 130 168 217 Porosity (vol %) 4 2 10Conformability to Irregularities/3-point bending test Good Good Bad JISK 7171 (2008) Boron Nitride Orientation Angle (α)(degrees) 15 16 16Particle g*^(A): Blended Weight [vol %]*^(B): Percentage relative to thetotal volume of the thermal conductive sheet (excluding curing agent)[vo l%]*^(C): Percentage relative to the total volume of the thermalconductive sheet Number of Time*^(D): Number of times of hot-pressing oflaminated sheet

TABLE 3 Average Particle Size Example (μm) Ex. 10 Ex. 11 Ex. 12 Ex. 13Ex. 14 Mixing Boron Nitride PT-110*¹ 45 3.83 13.42 13.42 13.42 13.42Formulation Particle/g*^(A)/ [40] [70] [70] [70] [70] of [vol %]*^(B)/[37.7] [69] [69] [69] [69] Components [vol %]*^(C) UHP-1*²  9 — — — — —Polymer Thermosetting Epoxy Resin Epoxy Resin A*³ 3 3 3 3 3 ResinComposition (Semi-solid) Epoxy Resin B*⁴ — — — — — (Liquid) Epoxy ResinC*⁵ — — — — — (Solid) Epoxy Resin D*⁶ — — — — — (Solid) Curing Agent*⁷ 63 3 3 3 (Solid Content in (0.3) (0.15) (0.15) (0.15) (0.15) Grams)Curing Agent*⁸ — — — — — (Solid Content in Grams) ProductionHot-pressing Temperature (° C.) 80 60 70 80 80 Conditions Number of Time(times)*^(D) 5 5 5 5 5 Load (MPa)/(tons) 20/5 20/5 20/5 20/5 40/10Evaluation Thermal Thermal Conductivity Plane Direction (SD) 4.1 10.511.2 32.5 50.7 Conductive (W/m · K) Thickness Direction 1.1 2.2 3.0 5.57.3 Sheet (TD) Ratio 3.7 4.8 3.7 5.9 6.9 (SD/TD) Glass Transition Point(° C.) 145 138 138 139 139 Porosity (vol %) 0 29 26 8 3 Conformabilityto Irregularities/3-point bending test Excellent Excellent ExcellentExcellent Good JIS K 7171 (2008) Boron Nitride Orientation Angle(α)(degrees) 20 17 15 15 13 Particle g*^(A): Blended Weight [vol%]*^(B): Percentage relative to the total volume of the thermalconductive sheet (excluding curing agent) [vol %]*^(C): Percentagerelative to the total volume of the thermal conductive sheet Number ofTime*^(D): Number of times of hot-pressing of laminated sheet

In Tables 1 to 3, values for the components are in grams unlessotherwise specified.

In the rows of “boron nitride particles” in Tables 1 to 3, values on thetop represent the Blended Weight (g) of the boron nitride particles;values in the middle represent the volume percentage (vol %) of theboron nitride particles relative to the total volume of the solidcontent excluding the curing agent in the thermal conductive sheet (thatis, solid content of the boron nitride particles and epoxy resin); andvalues at the bottom represent the volume percentage (vol %) of theboron nitride particles relative to the total volume of the solidcontent in the thermal conductive sheet (that is, solid content of boronnitride particles, epoxy resin, and curing agent).

For the components with “*” added in Tables 1 to 3, details are givenbelow.

PT-110*¹: trade name, plate-like boron nitride particles, averageparticle size (light scattering method) 45 μm, manufactured by MomentivePerformance Materials Inc.UHP-1*²: trade name: SHOBN®UHP-1, plate-like boron nitride particles,average particle size (light scattering method) 9 μm, manufactured byShowa Denko K. K.Epoxy Resin A*³: OGSOL EG (trade name), bisarylfluorene epoxy resin,semi-solid, epoxy equivalent 294 g/eqiv., softening temperature (ringand ball test) 47° C., melt viscosity (80° C.)1360 mPa·s, manufacturedby Osaka Gas Chemicals Co., Ltd.Epoxy Resin B*⁴: jER® 828 (trade name), bisphenol A epoxy resin, liquid,epoxy equivalent 184 to 194 g/eqiv., softening temperature (ring andball test) below 25° C., melt viscosity (80° C.) 70 mPa·s, manufacturedby Japan Epoxy Resins Co., Ltd.Epoxy Resin C*⁵: jER® 1002 (trade name), bisphenol A epoxy resin, solid,epoxy equivalent 600 to 700 g/eqiv., softening temperature (ring andball test) 78° C., melt viscosity (80° C.) 10000 mPa·s or more(measurement limit or more), manufactured by Japan Epoxy Resins Co.,Ltd.Epoxy Resin D*⁶: EPPN-501HY (trade name), triphenylmethane epoxy resin,solid, epoxy equivalent 163 to 175 g/eqiv., softening temperature (ringand ball test) 57 to 63° C., manufactured by NIPPON KAYAKU Co., Ltd.Curing Agent*⁷: a solution of 5 mass % Curezol® 2PZ (trade name,manufactured by Shikoku Chemicals Corporation) in methyl ethyl ketone.Curing Agent*⁸: a dispersion liquid of 5 mass % Curezol® 2P4 MHZ-PW(trade name, manufactured by Shikoku Chemicals Corporation) in methylethyl ketone.

While the illustrative embodiments of the present invention are providedin the above description, such is for illustrative purpose only and itis not to be construed as limiting the scope of the present invention.Modification and variation of the present invention that will be obviousto those skilled in the art is to be covered by the following claims.

1. A thermal conductive sheet comprising a plate-like boron nitrideparticle, wherein the thermal conductivity in a direction perpendicularto the thickness direction of the thermal conductive sheet is 4 W/m·K ormore, and a glass transition point determined as the peak value of tanδobtained by measuring a dynamic viscoelasticity of the thermalconductive sheet at a frequency of 10 Hz is 125° C. or more.